@interface ViewController : UIViewController <UIAlertViewDelegate,
                                              GKPeerPickerControllerDelegate>

- (void) receiveData:(NSData *)data fromPeer:(NSString *)peer
           inSession: (GKSession *)session context:(void *)context {
     // ...
}

grep -r receiveData /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS5.1.sdk